1. Field of the Invention
The invention relates to a method of determining a nuclear magnetization distribution or a distribution derived therefrom by subtraction of image information from images originating from a first and a second collection of resonance signals excited in a body, which is located in a stationary homogeneous magnetic field, at least comprising a first cycle of excitation sequences for producing the first collection of resonance signals, which comprises at least one high-frequency electromagnetic pulse for excitation of spin nuclei in the body and at least one magnetic field gradient superimposed on the homogeneous magnetic field, which each first sequence is repeated several times while varying the strength and/or duration of at least one magnetic field gradient, and at least comprising a second cycle of sequences for producing the second collection of resonance signals, which second sequence is repeated several times and differs from the first sequence in at least one gradient wave form for differently influencing in the first and in the second cycle moving spin nuclei and for influencing substantially equally stationary spin nuclei in the body, a first and a second complex image being formed by Fourier transformation of respective sample values of the respective resonance signals.
The invention further relates to an apparatus for determining a nuclear magnetization distribution of a body, which apparatus comprises means for producing a stationary homogeneous magnetic field, means for producing high-frequency electromagnetic pulses, means for producing magnetic field gradients, control means for the aforementioned means and processing means provided with programmed calculation means for forming complex images from sample values, which are obtained by sampling means from resonance signals produced in cycles in the body, the processing means further being provided with programmed calculation means for forming the nuclear magnetization distribution by subtraction of image information from images obtained while applying magnetic field gradients having different gradient wave forms.
2. Prior Art
Such a method for determining a nuclear magnetization distribution is particularly suitable for MR angiography, in which mostly a comparatively thick slice of the body is excited, of which, for example, a projective image of vascular trees is produced. Further, a flow speed distribution in the vessel can be determined.
Such a method is known from the article by D. G. Nishimura et al in "I.E.E.E. Transactions on Medical Imaging", Vol. MI-5, No. 3, September 1986. In FIG. 10b of the said article, as the first cycle a cycle is shown with a non-selective high-frequency electromagnetic pulse, a succeeding preparation gradient for encoding in phase in one direction the excitated spin nuclei and a bipolar read-out gradient for producing a resonance signal. The bipolar read-out gradient is switched so that the phase variation caused by movement is at least substantially zero. Page 141 of the said article states the condition on which the phase of moving spin nuclei at the maximum of the resonance signals can be made equal to zero, while assuming that the spin nuclei move at a constant speed in the direction of a gradient. In order to obtain the first collection of resonance signals, the preparation gradient is varied. FIG. 10a shows a second cycle, in which the read-out gradient is switched so that the phase variation caused by movement is different from zero. Further, the second cycle is identical to the first cycle. The second collection of resonance signals is now obtained. Therefore, in the first cycle speed is compensated for while this is not the case in the second cycle. By subtraction of complex image values in images obtained from the first and second collections by Fourier transformation, an angiogram is obtained. The angiogram shows vessels in which a flow speed component in the direction of the read-out gradient had occurred. At the area of moving spin nuclei, the phase is discontinuous. The phase differences caused by movement locally occur abruptly in voxels (volume elements) comprising a part of a blood vessel. The said article further describes the dependence upon position, speed and acceleration of the phase of the MR signal in a reconstructed complex image in dependence upon the gradient wave forms, also for gradients other than the read-out gradient. In the method described, for both collections the resonance signals in the same part of a heart cycle can be measured in order to avoid movement artefacts by the use of a sensor for eensing a heart signal. It should be noted here that the sensitivity of the magnetic resonance for flow has already been known for a long time, for example from the article by Hahn in "Physical Review". Vol. 80, No. 4, 1950, p. 580-594.
A disadvantage of the known method is that the image obtained by subtraction contains, besides information about vessels or about a speed distribution over the vessel, further background information originating from signals of stationary spin nuclei. This is due to the phase difference between the signals of stationary spin nuclei in the first and second cycles. This phase difference is formed due to the different eddy currents produced by the different gradient wave forms in the first and second cycles. The eddy currents produced by switching gradients will induce a smoothly varying phase over the images. The phase varies smoothly if the body is not situated too close to metal parts, in which the eddy currents flow. In general, this condition is fulfilled. The difference in gradient wave forms will therefore result in a smoothly varying contribution to the phase difference between the first and the second complex images. This becomes manifest in the said background information of stationary spin nuclei in the image obtained by subtraction.